1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases.
2. Description of the Prior Art
It is known from the U.S. Pat. No. 3,565,830 that a catalyst support for purifying exhaust gases can be modified to a better catalyst support by forming activated alumina coating layer on the catalyst support.
Further, it is known from Japanese Patent Laid-open Publications No. 63133/1982 that a catalyst for purifying exhaust gases comprising an inorganic porous support on which double oxide of cerium (Ce) and lanthanum (La) is formed, and further on which elements of the platinum group are carried on the double oxide.
The latter catalyst of aforesaid prior arts is intended to improve the exhaust gas purification ratio and the durability by enhancing the O.sub.2 (oxygen) storage effect and enlarging window. The window means a characteristic specifying a tolerance range of an excess air ratio (1/R) expressed in the following equation: EQU Excess air ratio (1/R)=O.sub.o /O.sub.R
where
O.sub.R : required oxygen amount for oxidizing all reduction compositions into water (H.sub.2 O) and carbon dioxide (CO.sub.2) PA1 O.sub.o : suppliable oxygen amount in oxidation compositions
The large window means that the tolerance range is wide. The exhaust gas purification ratio and the durability of the catalyst has been improved compared with a catalyst comprising cerium oxide alone.
However, it is desired that a catalyst decreases harmful components of exhaust gases such as hydrocarbons HC, carbon monoxide CO and nitrogen oxides NO to zero and that a catalyst should have a superior durability in view of the resource saving and the cost reduction requirements. Therefore, it has been required to develop a catalyst having superior exhaust gas purifying property and durability to aforesaid catalysts.